But then one could just as happily ask why call it an Orc-zombie. One term is as baseless as the other.

The relevant bits of Morgoth's Ring are several essays (Myths Transformed) in which Tolkien discusses conceptions of the nature of Orcs, amongst other things. These are some of Tolkien's last recorded thoughts on the matter.

It is of course perfectly reasonable to take the position that what Tolkien published should be considered to have primacy. It is also equally valid to consider that his ideas developed over time and that later ideas supercede prior ones. In either case I think they are well worth exploring.

I'm a bit puzzled by the idea, in principle, that drawing on a wider knowledge of Tolkien's ideas for the adaptation of a particular work is "unacceptable" per se.